Auxins are growth regulators involved in virtually all aspects of plant development. For example, apical dominance, cell expansion, vascular differentiation, lateral root and root hair formation, phototropism and root gravitropism are among the many processes in plants controlled by auxins (DAVIES P. J. (1995) In Plant Hormones Kluwer Academic Publishers, Dordrecht, The Netherlands, pp 1–12). The level of auxin is regulated by both de novo biosynthesis and reversible and irreversible conjugation to sugars, amino acids and peptides as well as by degradation. Though the chemical structure of the primary auxin, indole-3-acetic acid (IAA), has been known since the 1930's (Wildman S. G. (1997) Plant Growth Reg. 23:37–68) not much is known about how plants actually synthesize this essential compound. Plants appear to be capable of synthesizing IAA by both tryptophan-dependent and tryptophan-independent pathways. Classical incorporation studies with radiolabeled compounds have not unambiguously identified either the precursors nor elucidated the biosynthetic pathway for IAA. (For a recent review on IAA metabolism, see Normanly and Bartel (1999) Curr. Opin. Plant Biol. 2:207–213.)
Although a number of mutants in IAA metabolic pathways and perception have been described, the genes involved and their biochemical function and physiological relevance have not all been elucidated (reviewed in Bartel B. (1997) Ann. Rev. Plant Physiol. Plant Mol. Biol. 48:51–66; Normanly and Bartel (1999) Curr. Opin. Plant Biol. 2:207–213). For example, the rty/surl/hls3/alfl and sur2 mutants are both known to accumulate increased levels of free auxin. Identification of the proteins or gene products affected and elucidation of the biochemical roles of these genes/proteins should increase the limited knowledge of IAA biosynthesis and regulation.
Glucosinolates are sulfur containing bioactive natural products derived from amino acids and sequestered in vacuoles of cruciferous plants (Selmar (1999) In Biochemistry of Plant Secondary Metabolism. Annual Plant Reviews 2:79–150). It has recently been shown that the cytochromes CYP79B2 and CYP79B3 of Arabidopsis thaliana both metabolize tryptophan to indole-3-acetaldoxime. This metabolite has been suggested to be the precursor of indole-3-acetonitrile (IAN) in IAA biosynthesis (Normanly and Bartel (1999) Curr. Opin. Plant Biol. 2:207–213; Hull et al. (2000) Proc. Natl. Acad. Sci. USA 97:2379–2384), as well as of thiohydroximates in glucosinolate biosynthesis, though neither step has been characterized biochemically. Nitrilases that catalyze the conversion of IAN to IAA are well characterized in Arabidopsis (Bartel and Fink (1994) Proc. Natl. Acad. Sci. USA 91:6649–6653). In this species, four differentially regulated nitrilases have been identified, though their physiological role is not clear (Normanly and Bartel (1999) Curr. Opin. Plant Biol. 2:207–213). A mutation for one of the nitrilase genes, nit1, renders Arabidopsis seedlings insensitive to exogenously applied IAN, yet this mutant does not have an apparent physiological IAA phenotype under normal conditions. (Normanly et al. (1993) Proc. Natl. Acad. Sci. USA 90:10355–10359).
Thus, there remains a need for the identification and characterization of enzymes that regulate auxin and glucosinolate synthesis.